Sram bus architecture and interconnect to an fpga

ABSTRACT

An SRAM bus architecture includes pass-through interconnect conductors. Each of the pass-through interconnect conductors is connected to routing channels of the general interconnect architecture of the FPGA through an element which includes a pass transistor connected in parallel with a tri-state buffer. The pass transistors and tri-state buffers are controlled by configuration SRAM bits. Some of the pass-through interconnect conductors are connected by programmable elements to the address, data and control signal lines of the SRAM blocks, while other pass through the SRAM blocks with out being further connected to the SRAM bussing architecture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/410,415, filed Apr. 24, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/802,577, filed on Mar. 16, 2004, now issued asU.S. Pat. No. 7,054,967, which is a continuation of U.S. patentapplication Ser. No. 10/406,860, filed Apr. 4, 2003, now issued as U.S.Pat. No. 6,799,240, which is a continuation of U.S. patent applicationSer. No. 10/247,114, filed Sep. 18, 2002, now issued as U.S. Pat. No.7,146,441, which is a continuation of U.S. patent application Ser. No.09/512,133, filed Feb. 23, 2000, now issued as U.S. Pat. No. 6,496,887,which is a continuation of U.S. patent application Ser. No. 09/039,923,filed Mar. 16, 1998, now issued as U.S. Pat. No. 6,038,627, the entiretyof which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a field programmable gate array(FPGA) having embedded static random access memory (SRAM). Moreparticularly, the present invention is related to a bus architecture forthe embedded SRAM, and the connection of the SRAM bus architecture tothe general interconnect architecture of the FPGA.

2. The Prior Art

As integrated circuit technology advances, geometries shrink,performance improves, and densities increase. This trend makes thedesign of systems of ever increasing complexity at ever decreasing costfeasible. This is especially true in logic products such as ApplicationSpecific Integrated Circuits (ASICs), Complex Programmable Logic Devices(CPLDs), and Field Programmable Gate Arrays (FPGAs).

The need for integrating fast, flexible, inexpensive memory into theselogic products to provide memory for a variety of purposes such asregister files, FIFOs, scratch pads, look-up tables, etc. has becomemore apparent, because there are significant cost and performancesavings to be obtained by integrating this functionality directly into,for example, an FPGA. However, providing this memory by having somethingother than explicitly dedicated SRAM blocks included in the FPGA has notproved satisfactory. Typically, the implementation of memory withoutdedicated SRAM blocks in an FPGA has been done by either providingexternal SRAM to the FPGA or by using the logic modules, flip-flops andinterconnect of the FPGA. Both of these solutions are less thansatisfactory.

Using external SRAMs with FPGA designs is undesirable for severalreasons. Separate memory chips are expensive, require additional printedcircuit board space, and consume I/O pins on the FPGA itself. Also, aseparate memory chip is required to implement each memory function,thereby further increasing the cost.

When SRAM is implemented with the logic modules in the FPGA, it requiresa substantial amount of the routing and logic resources of the FPGA,because the available logic blocks are implemented as gates and latchesand the programmable interconnect is employed to connect them. Thissubstantially degrades both the performance and flexibility of the FPGAby consuming a considerable amount of logic array resources, and imposescritical paths that are quite long for even a small memory block.

Xilinx offers the capability of using the configurable logic blocks ontheir 4000 Series of parts as 16×1 SRAM blocks, but requires the use ofnormal interconnect to combine the blocks into larger memoryconfigurations. While this distributed SRAM approach is an improvementin density and is flexible for building larger memories, it is stillslow and consumes logic array resources. The necessary overheadcircuitry was sufficiently large that Xilinx actually removed it whenthey developed their low cost 4000-D parts. On their 4000-E Seriesparts, they offer the ability to configure two configurable logic blocksto emulate a dual ported 16×1 SRAM block, however, this design stillcarries with it performance and flexibility degradation.

Altera has also attempted to improve on the connection of the SRAMblocks in their embedded array blocks for their 10K FLEX parts. Theyinclude one or more columns on their larger parts of embedded arrayblocks, which are size matched to their logic array blocks. The embeddedarray blocks contain 2K bits of single ported SRAM configurable as256×8, 512×4, 1024×2, or 2048×1. This approach builds the flexibility ofdifferent widths and depths into the SRAM block, but at a significantperformance cost since the access time of an embedded array block isvery slow for a memory of the size and the technology in which it isbuilt. Further, array routing resources are required for memoryconfigurations other than those indicated.

Another approach to SRAM memory in FPGA applications is found in“Architecture of Centralized Field-Configurable Memory,” Steven J. E.Wilton, et al., from the minutes of the 1995 FPGA Symposium, p. 97. Thisapproach involves a large centralized memory, which can be incorporatedinto an FPGA. The centralized memory comprises several SRAM arrays,which have programmable local routing interconnect which are usedexclusively by the centralized memory block. The local routinginterconnects are used to make efficient the configuration of the SRAMswithin the centralized memory block. However, the local interconnectstructure disclosed in Wilton suffers performance problems due toexcessive flexibility m the interconnect architecture.

Actel's 3200 DX family of parts attempted an intermediate approach byincluding columns of dual ported SRAM blocks with 256 bits, which areconfigurable as either 32×8 or 64×4. These blocks are distributed overseveral rows of logic modules to match the density of I/O signals to theSRAM block to that of the surrounding FPGA array. Polarity controlcircuits were added to the block enable signals to facilitate use ashigher address bits. This architecture was designed to provide highperformance and reasonable flexibility, with density approaching theinherent SRAM density of the semiconductor process, and routing densitycomparable to the rest of the logic array. Unfortunately, this approachrequired array routing resources to interconnect SRAM blocks into deeperand wider configurations.

One of the desirable attributes of user-assignable SRAM blocks in anFPGA architecture is the ability to connect the SRAM blocks to oneanother to form memories that are either wider (i.e. longer word length)or are deeper (i.e. more words). In connecting SRAM blocks into deeperand wider configurations it must be appreciated that the addresses haveto go to each of the SRAM blocks, the write data has to go to each ofthe SRAM blocks, and the data must be able to be read from all of theSRAM blocks. In addition, the control signals used by the SRAM blocks toread and write data must also be routed to each of the SRAM blocks.

Since routing resources must be used to connect the dedicated SRAMblocks to one another to create either wider or deeper memories, andgiven that routing resources are not unlimited, preventing a degradationin the performance of the FPGA by efficiently forming deeper and widermemories is an important concern. In preventing a degradation of theFPGA performance, the connection to the user of SRAM blocks to providedeeper and wider memory configurations should not substantially impactthe place and route algorithms of the FPGA, nor prevent the use of placeand route algorithms for connecting the logic in the FPGA. Severalapproaches are known in the art for configuring dedicated SRAM blocks toprovide deeper and wider memories.

The difficulty in creating deeper arid wider SRAM block configurationsin the prior art has been that array routing resources have beenrequired to interconnect the SRAM blocks into these configurations. Partof the problem has been that the array routing resources have not beenused very efficiently. In certain instances, this was due to the factthat the devices to which the SRAM blocks have been added were notoriginally designed with embedded SRAM blocks, rather the SRAM blockshave been inserted as an add-on piece.

These problems are better illustrated with reference to FIGS. 1 and 2.In FIG. 1, four 256×8 SRAM blocks are connected into a deeperconfiguration or essentially a 1024×8 memory. In this configuration itcan be seen that the lower order write address bits must be supplied toeach of the 256×8 SRAM blocks along with the write data. Additionally,logic must be implemented to provide a 2 to 4 decode of the two higherorder address bits used to select the one of four 256×8 SRAM blocks towhich the data will actually be written. To read data from the SRAMblocks in this deeper configuration, the lower order read address bitsmust be supplied to each of the 256×8 SRAM blocks, and then additionallogic must be implemented to provide a 4 to 1 multiplexer so that thecorrect data may be selected from the 1 of 4 SRAM blocks from which datais being output.

In FIG. 2, four 256×8 SRAM blocks are configured in a widerconfiguration to provide a 256×32 SRAM block. In this configuration, thewrite address must be supplied to each of the 256×8 blocks to perform awrite operation, and to perform a read operation the read address mustalso be supplied to each of the 256×8 blocks. The write data must berouted so that the first 8 bits of the data at a particular addresslocation is supplied to a first 256×8 SRAM block, the next 8 bits of thedata at that particular address location is supplied to a second 256×8SRAM block, and the third and fourth 8 bits of data at the same addressare supplied to a third and fourth SRAM block, respectively. For a dataread, the output of the SRAM blocks must be connected so that thecorrect 8 bits are taken from each of the four 256×8 SRAM blocks to forma single 32 bit word.

Clearly there is a need for a flexible SRAM bus architecture in an FPGAfor embedding user-assignable SRAM blocks into the FPGA. The SRAM busarchitecture of the present invention provides routing resources forefficiently using individual SRAM blocks or for connecting multipleblocks of user SRAM to make wider and/or deeper memory configurations.The SRAM bus architecture of the present invention is implemented withminimal use of the routing resources of the array, and with a minimaldegradation in the performance of the FPGA.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention, an SRAM bus architecture isdisclosed that is suitable for use with a plurality of embedded SRAMblocks m an FPGA. The SRAM bus architecture is employed to connect theSRAM blocks in a group together into deeper and wider configurationswithout employing the general interconnect architecture resources of theFPGA, and to connect the SRAM blocks to the logic modules in an FPGA.

The SRAM bus architecture-includes pass-through interconnect conductors.Each of the pass-through interconnect conductors is connected to therouting channels of the general interconnect architecture of the FPGAthrough an element which includes a pass transistor connected inparallel with a tri-state buffer. The pass transistors and tri-statebuffers are controlled by configuration SRAM bits. Some of thepass-through interconnect conductors are connected by programmableelements to the address, data and control signal lines of the SRAMblocks, while other pass through the SRAM blocks with out being furtherconnected to the SRAM bussing architecture. This aspect of the presentinvention, increases the efficiency of the place and route of the FPGA.

The SRAM blocks are dual ported (simultaneous read/write), and includean additional load port that interacts with the circuitry employed inthe loading and testing of the configuration data of the FPGA core. EachSRAM block contains circuits in both the read port and the write portthat together with the SRAM bus architecture permit the SRAM blocks tobe connected into deeper and wider configurations by without anyadditional logic as required by the prior art.

In the preferred embodiment, there eight fully independent blocks of 2Kbit SRAM blocks, wherein each SRAM block is organized as 256 words of 8bits each. The eight SRAM blocks are further divided into two groupssuch that the SRAM blocks in each of the groups are substantiallycontiguous to the extent that the address busses, data busses, andcontrol signal lines of each of the user-configurable SRAM blocks in agroup can be commonly connected by user-programmable elements at theiredges to facilitate directly combining the user-configurable SRAM blocksm a group into wider and/or deeper user-assignable memoryconfigurations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the connection of four 256×8 SRAM blocks into adeeper memory configuration according to the prior art.

FIG. 2 illustrates the connection of four 256×8 SRAM blocks into a widermemory configuration according to the prior art.

FIG. 3 illustrates in block diagram an SRAM based FPGA core according tothe present invention.

FIG. 4 illustrates a hierarchical embodiment of groups of logic modulesaccording to the present invention.

FIG. 5 illustrates in block diagram the logic entities included in alogic module suitable for use according to the present invention.

FIG. 6 illustrates block diagram an SRAM block depicting a write port,read port and load port suitable for use according to the presentinvention.

FIG. 7 is a schematic diagram of the connection of the interconnectarchitecture of an MLA3 to the SRAM bus architecture according to thepresent invention.

FIGS. 8A-8C illustrate programmable connections according to the presentinvention.

FIG. 9 is a circuit diagram illustrating the enable logic in the writeport of the SRAM blocks according to the present invention.

FIG. 10 illustrates a polarity select circuit suitable for use in thepresent invention.

FIG. 11 illustrates high impedance and fast access output features ofthe read port in each of the SRAM blocks according to the presentinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons.

In the present invention, the SRAM bus architecture associated with theSRAM blocks is distinct from the general interconnect architecture ofthe FPGA. As will be described below, a distinct SRAM bus architectureprovides advantages in connecting the SRAM blocks to the other portionsof the FPGA not contemplated by the prior art. The SRAM bus architectureof the present invention does not require the use of the routingresources of the general interconnect architecture of the FPGA toconnect SRAM blocks together in the manner known in the prior art, andcan make use of the overhead generally allocated in SRAM blocks for theformation of address busses, data busses, and control lines.

Turning now to FIG. 3, a block diagram of the FPGA core 10 in aflexible, high-performance SRAM based FPGA architecture havinguser-assignable SRAM blocks 12 according to the present invention isillustrated. The FPGA core 10 includes a plurality of logic functionmodules disposed within a multi-level architecture (MLA) of routingresources. The blocks 14-1 through 14-4 in FPGA core 10 illustrate agrouping of logic modules in the MLA termed MLA4. In the preferredembodiment, there are approximately 50K gates combined in blocks 14-1through 14-4. The SRAM blocks 12 comprise 16K bits of user-assignableSRAM divided into eight dedicated blocks of 2K bits. The user-assignableSRAM blocks 12 will be more fully described below.

Depicted in FIG. 4, are the relative number of logic modules included ineach of the routing resource levels MLA1, MLA2, MLA3, and MLA4 in theMLA. In the grouping of logic modules in the MLA, each level in the MLAincludes four groups of logic modules from the next lower level in theMLA. The highest level in the MLA is MLA5 comprising all of the logicmodules in the FPGA core 10, the SRAM blocks 14, the configuration SRAM,and other circuits in the FPGA.

The smallest grouping of logic modules is a level MLA1 in the MLA. EachMLA1 level includes four blocks each having four clusters. Each of theclusters can be considered as a logic module having four separate logicentities. FIG. 5 illustrates the logic included in a cluster 16. Thecluster 16 includes first and second three-input configurable functiongenerators (CFG) 18 and 20, respectively, a two-input CFG 22, and aD-type flip-flop 24. With a three input CFG any three input booleanlogic function may be implemented, and with a two input CFG any twoinput boolean logic function may be implemented. To avoidovercomplicating the disclosure and thereby obscuring the presentinvention, many of the details of the entire MLA are not disclosedherein.

Referring again to FIG. 3, each user-assignable SRAM block 12 includes256 words of 8 bits, and each user-assignable SRAM block 12 is fullyindependent from the other SRAM blocks 12. The eight bits of addressingfor each word in an SRAM block 12, and the eight bit word length areconvenient for connection to the MLA4 blocks 14-1 through 14-4. It willbe appreciated by persons of ordinary skill in the art that SRAM blocks12 which are either larger or smaller than 2 k bits may be utilized, andfurther that either fewer or greater than eight SRAM blocks 12 may beemployed. The number of words in an SRAM block 12, the length of a wordin an SRAM block 12, the number of bits in an SRAM block 12, and thenumber of SRAM blocks 12 are a matter of design choice.

In the FPGA core 10, the SRAM blocks 12 are preferably split into twogroups of four SRAM blocks 12. It should be appreciated that the numberof SRAM blocks in a group is a matter of design choice. A first group offour SRAM blocks 12 is disposed between MLA4 blocks 14-1 and 14-3, and asecond group of four SRAM blocks 12 is disposed between MLA4 blocks 14-2and 14-4. The SRAM blocks 12 in each of the groups are substantiallycontiguous to the extent that the address busses, data busses, andcontrol signal lines of each of the SRAM blocks 12 in a group can becommonly connected to facilitate directly combining the SRAM blocks 12in a group into wider and/or deeper user-assignable memoryconfigurations. Further, the two groups can be connected together, ifdesired, through the logic modules and the general interconnectresources of the MLA.

Referring now to FIG. 6, each SRAM block 12 is depicted as dual portedSRAM having write and read ports 26 and 28, respectively. Connected towrite port 26 are an eight-bit write address (WA) bus, an eight-bitwrite data (WD) bus, a write clock (WCLK) input, and a three-bit writeenable (WEN) bus. Connected to the read port 22 are an eight-bit readaddress (RA) bus, an eight-bit read data (RD) bus, a read clock (RCLK)input, a three-bit read enable (REN) bus, a three-bit read output enable(ROE) bus, and read output enable control (ROEC) input. Each of thecontrol signal lines namely, WCLK, WEN. RCLK, REN, ROE, ROEC, have awell known in-line inverter, comprising an XNOR gate and a polaritycontrol bit as is well known in the art and as is described below withrespect to FIG. 10, that may be selected to provide a connected signalor its complement.

To avoid over complicating the present disclosure and obscuring thepresent invention only some of the details of the SRAM blocks 12 will bedisclosed herein. A more detailed description of the SRAM blocks 12 aredisclosed in U.S. Pat. No. 6,049,487, assigned to the assignee of thepresent invention and specifically incorporated herein by reference.

Also depicted in FIG. 6 is a load port 30 having connected thereto acolumn address bus, a row address bus, an LD line, and an LDB line. Theload port 30 is controlled by the circuitry employed to load theconfiguration data into the configuration SRAM of the FPGA core 10. Toavoid over complicating the present disclosure and obscuring the presentinvention, the details of the circuitry employed in the loading andtesting of the configuration data of the FPGA core 10 will not bedisclosed herein. These details are disclosed in U.S. Pat. No.6,237,124, assigned to the assignee of the present invention andspecifically incorporated herein by reference.

Referring now to FIG. 7, a more detailed block diagram of a portion ofthe FPGA core 10 depicts the interconnectivity between the SRAM blockbus architecture and the routing channels of an MLA3 32. In FIG. 7,connectors 34 between an MLA3 32 and the SRAM blocks 12 are disposed onthe edge of an MLA3 32 adjacent an SRAM block 12. Each of the connectors34 represents a plurality of user-programmable interconnect elementsdisposed between the conductors of a routing channel in an MLA3 32 and aplurality of pass-through interconnect conductors 36 spanning the SRAMblock 12.

It is presently contemplated that each MLA4 14 will have sixteenconnectors 34 on the edge abutting SRAM blocks 12. As depicted in FIG.3, the width of an MLA4 14 is such that two SRAM blocks 12 will fitalong the side of an MLA4 14. Accordingly, eight connectors 34 in anMLA3 32 will abut each SRAM block 12 as illustrated in FIG. 7. Of theseeight connectors 34, some are employed to make selective connections topass-through interconnect conductors 36 that are not further connectableto the SRAM bussing architecture, while the remaining block connectors34 are employed to make selective connections to pass-throughinterconnect conductors 36 that are further connectable to the SRAMbussing architecture.

Selective connections are made between the interconnect conductors inthe routing channels of the MLA3 32 and the pass-through interconnectconductors 36 by the user-programmable interconnect conductors of theconnectors 34. The user programmable interconnect elements of theconnectors 34 is preferably a pass transistor connected in parallel witha tri-state buffer as illustrated in FIG. 8A.

The WA, WD, WCLK, WEN, RA, RD, RCLK, REN, ROE, and ROEC signal lines ofthe SRAM bus architecture all horizontally traverse each of the SRAMblocks 12 to form intersections with the pass-through interconnectconductors 36. The address, data and control lines and are selectivelyconnectable by user-programmable interconnect elements 38 to thepass-through interconnect conductors 36. The user-programmableinterconnect elements 38 are preferably pass gates controlled by SRAMconfiguration bits disposed between the address, data and control linesand the pass-through interconnect conductors 36 in a manner depicted inFIG. 8B. Those of ordinary skill in the art will appreciate that theprogrammable elements may also include other programmable elements knownin the art, further including, antifuses, E²ROM bits, etc.

The intersections of the address, data and control signal lines in theSRAM bus architecture with the pass-through interconnect conductors 36may be less than fully populated with user-programmable interconnectelements 38. As illustrated in FIG. 7, user-programmable interconnectelements 38 are disposed at the intersections of the address and datalines and a first group of pass-through interconnect conductors 36, anduser-programmable interconnect elements 38 are also disposed at theintersections of the control signal lines and a second group of passthrough interconnect conductors 36. It should be appreciated that any ofthe pass through interconnect conductors 36 may form intersections withboth address and data lines and also control signal lines at which aredisposed user-programmable interconnect elements 38. It will beappreciated that the intersections of the address, data and controlsignal lines and the pass-through interconnect conductors 36 at whichare disposed the user-programmable interconnect elements 38 is a matterof design choice.

In FIG. 7, user-programmable interconnect elements 40, which arepreferably pass transistors, are disposed in series with thepass-through interconnect conductors 36 at the edges of the SRAM blocks12, and also in series with the address, data and control signal linesat the edge of two SRAM blocks 12 in the same group. A pass transistordisposed in series with two conductors is illustrated in FIG. 8C.

According to the present invention, each SRAM block 12, containscircuits in both the read port and the write port that together with theSRAM bus architecture disclosed herein permit the SRAM blocks 12 to beconnected into deeper and wider configurations by without any additionallogic as required by the prior art. Referring now to FIGS. 9, 10, and 11the portions of the SRAM blocks 12 to which the control lines and highimpedance controls employed to connect SRAM blocks 12 into wider anddeeper configurations are shown in greater detail.

In FIG. 9, four SRAM blocks 12 in a group are illustrated. The enablelogic 60 in the write port 26 of each of the SRAM blocks 12 to which isconnected the WEN control signals has been separated from the remainingportions 62 of the SRAM blocks 12 to better illustrate the manner inwhich the SRAM blocks 12 may be connected into deeper and widerconfigurations. Connected to each of the SRAM blocks 12 are three WENcontrol signals representing the eighth and ninth bits of a writeaddress, and a write enable signal. In each of the SRAM blocks 12, thethree WEN control lines are connected to polarity select circuits 64.

In the preferred embodiment of the present invention, each of thepolarity select circuits 64 comprises an exclusive NOR (XNOR) gate 66and a polarity control bit 68 as illustrated in FIG. 10. The operationof the exclusive XNOR gate 64 is well understood by those of ordinaryskill in the art. Depending upon the value of the polarity control bit68, the input to the exclusive XNOR gate 66 can either be passed throughthe exclusive XNOR gate 66 or complimented. The polarity control bit 68is provided as part of the configuration memory.

In each of the SRAM blocks 12, the AND gates 70 connected to the outputsof the polarity select circuits 64 form an AND decode of the WEN1 andWEN2 signals (the eighth and ninth address bits) as is well understoodby those of ordinary skill in the art. The output of each of the ANDgates 70 provides a write enable signal to the remainder of each of theSRAM blocks 62.

Turning now to FIG. 11, a simplified schematic of the connections of theREN, ROE, and the ROEC to an SRAM block 12, is illustrated. In theillustrative SRAM block 12, each of three REN signals are connected to apolarity select circuit 92 similar to the polarity select circuit 64shown in FIG. 10. The outputs from the polarity select circuits 92 areconnected to the input of an AND gate 94. As is well understood by thoseof ordinary skill in the art, the polarity select circuits 92 and ANDgate 94 form an AND decoder for the REN signals.

The output of the AND gate 94 is connected to a read control circuit 96in the read port 28 of SRAM block 12. One of the outputs of the readcontrol circuit 96 is connected to and controls the enable input tosense amplifiers 98. The sense amplifiers 98 are connected to the SRAMcells in the SRAM block 12. During a read operation, the senseamplifiers 98 sense the contents of the SRAM cells in the SRAM block.The output of the sense amplifiers 98 is connected to the inputs ofoutput drivers 100. The outputs of output drivers 100 are connected tothe RD bus. When desired, the output drivers 100 can be placed in a highimpedance condition by the output from AND gate 94 connected through afirst input of an OR gate 102 and a first input of an AND gate 104 tothe enable input of the output driver 100. By including the highimpedance feature for the RD bus by placing the output drivers 100 intoa high impedance condition, the SRAM blocks 12 can be connected intodeeper configurations without the need for external logic.

With the high impedance RD bus, a fast access mode of operation may beimplemented. Since enabling the output drivers 100 is significantlyfaster than an SRAM read operation, providing a fast enable to theoutput drivers 100 when a read has previously taken place in the SRAMblock 12 can create the illusion of a faster read access than isnormally possible. Thus in a deep multiple SRAM block configuration, aread can be conducted in all the SRAM blocks 12 in the configuration andthen deft manipulation of the output driver 100 enables can create aneffectively larger output bandwidth.

In FIG. 11 fast access circuitry is illustrated wherein a second inputof OR gate 102 is connected to the ROEC input through polarity selectcircuit 106. The output of OR gate 102 is connected to a first input ofAND gate 104 having second, third and fourth inputs connected to thethree-bit ROE bus through polarity select circuits 108. In the operationof the fast access mode, the ROEC input is used to force a logic-1 intothe first input of the AND gate 104 so the output buffer enable issolely under the control of the ROE bus. In normal operation, the ROECinput and the ROE bus are set up so that the output of the AND gate 94controls the output drivers 100.

In the present invention, the independence of the SRAM bus architectureassociated with the SRAM blocks 12 from the FPGA interconnectarchitecture has several distinct advantages over the connectivity ofprior art SRAM blocks to an FPGA interconnect architecture. As describedabove, in connecting the FPGA architecture to the SRAM bus architecture,some of the pass-through interconnect conductors 36 may be madeconnectable to the SRAM bus architecture, while others of thepass-through interconnect conductors 36 are not made connectable to theSRAM bus architecture.

In a first aspect, the pass-through interconnect conductors 36 notconnected to the SRAM bus architecture enhance the overall partconnectivity by providing connection paths between MLA4 14 that are notdirectly connectable to the SRAM bus architecture. This is an importantaspect of the present invention, since it permits interconnectionbetween an MLA4 14 located adjacent to one SRAM block 12 and an MLA4 14located adjacent another SRAM block 12 as if the SRAM blocks 12 were notpresent, thus rendering the SRAM blocks 12 virtually transparent to therouting resource.

In a second aspect, the pass-through interconnect conductors 36 that areselectively connectable to the SRAM bus architecture provide ampleconnection of the MLA4 14 to the SRAM blocks 12. Further, because FPGArouting resources are not used to route common address busses, databusses, and control lines between the SRAM blocks 12, the routingefficiency of the FPGA architecture will increase. It will beappreciated by those of ordinary skill in the art that the commonaddress busses, data busses, and control lines in the SRAM blocks 12require no additional integrated circuit space overhead, because spaceis already allocated generally for the formation of address busses, databusses, and control limes in SRAM blocks.

The FPGA architecture described herein offers flexible, high-performanceSRAM to the user of FPGAs. The flexibility of the architecture permitsefficient implementation of on-chip data storage, register files, andFIFOs. Small-capacity high-speed dual-port SRAM can be used to handleATM data packets; for DRAM and DMA control; as a “rubber-band”synchronizer between two clocks of differing frequency; and as acoefficient table for FIR and KR filters (wherein many integercoefficients are stored once and retrieved repeatedly).

By offering many independent blocks, the FPGA can support many differentsorts of applications. Unused blocks can be turned into 8-bit registers.On-chip SRAM is many times more efficient for storing data than logicmodules and saves many valuable I/O pins. Thus, the user can fit morelogic into, and obtain greater performance from, a given FPGA.

Those of ordinary skill in the art will recognize that the SRAMarchitecture disclosed herein can also be utilized for FIFO, ROM, and assingle port RAM with or without employing a bidirectional data bus.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The invention, therefore, is not tobe restricted except in the spirit of the appended claims.

1. A bus architecture for a plurality of dedicated SRAM blocks in anFPGA comprising: a first edge of said FPGA having FPGA routingconductors connected to a first plurality of programmable connectors; asecond edge of said FPGA having FPGA routing conductors connected to asecond plurality of programmable connectors; a plurality of pass-throughinterconnect conductors, each of said plurality of said pass-throughinterconnect conductors having a first end connected to one of saidfirst plurality of programmable connectors and a second end connected toone of said second plurality of programmable connectors; an address busdisposed in one of said plurality of dedicated SRAM blocks forming firstintersections with said plurality of pass-through interconnectconductors; a data bus disposed in one of said plurality of dedicatedSRAM blocks forming second intersections with said plurality ofpass-through interconnect conductors; a control signal line disposed inone of said plurality of dedicated SRAM blocks forming thirdinterconnect intersections with said plurality of pass-throughinterconnect conductors; and programmable elements disposed at selectedones of said first, second and third intersections.